Tiger is basically correct but he's off by a year. The FX4 was introduced in 2002. The subject comes up at least monthly on the Ranger site where I'm a moderator. Sorta goes like this:
'98-'02 Rangers had an optional Off-Road Package (aka ORP). The content varied a little by year but basically you got skidplates, stiffer shocks, P245/75-16 tires, foglights, and decals. A limited-slip rear was NOT included but was optional. I had an '02 Ranger so equipped before I got my Trac.
An '02 FX4 also got you Bilstien shocks, a front skidplate, rear tow hook, Alcoa wheels with the 31" BFG A/T's, and a 31-spline 8.8" rear with a Torsen limited-slip and fatter axle tubes*. Also retina-searing upholstery.
'03-up the ORP became the FX4 with the added benefit of a regular limited-slip rear being included.
The '02 FX4 package became the FX4 Level II for '03-up and still gets the hootchie-mama Torsen rear diff.
Therefore:
'98-'02 ORP = '03-up FX4
'02 FX4 = '03-up FX4 Level II
* Rangers have a somewhat weaker 28-spline 8.8" rear. Exploders and Tracs have a stronger 31-spline unit. So you're thinking "Yah, I'd like that Torsen in my Trac." A Detroit True Trac diff is the same thing for somewhat less money, probably way less than the Ford parts counter, and it's extreeeemly unlikely that you'll find an FX4 Torsen in a junkyard (no offense, Coastiejoe). This is a gear-driven limited-slip and does not use clutches. I'm also told that Reider Racing can modify a True Trac to fit up front, important since I guess the Powertrax No-Slip for our front is no longer made. [/off-topic rant]